I don't recall Heimdall bestowing any power to Eric Masterson. Could you cite the issue you're referring to? The only thing close that I recall is that Heimdall said that it was only by his graces that Thor retained the power.

It should be noted the Heimdall only had a fraction of Odin's power. The bulk of it was held by Loki in Odin's body.

When Thor was released from Masteron's mind, and restored to corporeal form, Masterson and Thor seemed parallel in strength. Thor was out-skilling him, but in the end, Thor resorted to brute force. And Masterson held his own.